Abstract: The present study was to determine the effects of water temperature, nitrogen and phosphorus concentrations, vitamin and antibiotic concentrationson the growth of toxic Alexandrium tanarense(Hong Kong Chain ò-under experimental conditions.The strains of Alexandrium tamarense was isolated in 1998 from Hong Kong sea area where harmful algal blooms(HAB-had frequently occurred, some of which were highly toxic.The results showed that its optimum growth condition was at 21-25℃, 882-1765Lmol N/L and 18-72Lmol P/L respect ively.When temperature was 21℃, nitrogen concentration was 1765Lmol/L and phosphorus concentration was 72Lmol/L, the specific growth rate calculated from microscopic cell counts over the exponential phase of growth attained the higher value of 0.21, 0.28 and 0.28 d-1 respectively.Incorporated vitamin made up of B1, B6 and B12 had positive effect on the growth of Alexandrium tamarense when the contents of them ranged from 0, 0 and 0Lg/L to 500, 500 and 110Lg/L.The dinoflagellate got their maximal growth rate 0.26 d-1 at the highest vitamin concentrations.While the antib-i otic( liquid of ampicillin-exceeded 50U/mL had obviously inhibition on it.The cellule algae of Alexandrium tamarense was observed in the culture containing ant ibiotic, and the forming mechanism required extended studies.